page_title: Opisthobranchia
section_title: As a non-monophyletic taxon
hierarchical_section_title: Opisthobranchia / Taxonomy / As a non-monophyletic taxon
caption_attribution_description: Sea Hare, Morro Bay, CA, March 23, 2007. Shot with a Canon 20D with 100-400mm IS lens handheld from a kayak, shot in RAW, sharpened in Photoshop CS2. Photo by Mike Baird bairdphotos.com 23mar2007 23march2007 Identification: This is an Aplysia californica
context_page_description: Opisthobranchs is now an informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping.
  Euopisthobranchia is a taxon containing a revised collection of opisthobranchs, and that taxon is considered monophyletic. Euopisthobranchia does not include some "traditional" opisthobranchs such as the Sacoglossa and the Acochlidiacea. The subclass Heterobranchia now contains all the species which used to be assigned to Opisthobranchia, plus all the species in the Pulmonata.
  The subclass Opisthobranchia included species in the order Cephalaspidea, the sacoglossans, anaspidean sea hares, pelagic sea angels, sea butterflies, and many families of the Nudibranchia.
  Opisthobranch means "gills behind" of the heart. In contrast, Prosobranch means gills in front. Opisthobranchs are characterized by two pairs of tentacles and a single gill behind and to the right of the heart. Opisthobranchia are known from as early as the Carboniferous.
context_section_description: It was speculated as far back as 1985 that the Opisthobranchia were paraphyletic, based on morphological evidence, and had given rise to the Pulmonata, a group also of subclass rank.
  Because the Pulmonata are a sibling group to an opisthobranch taxon, some authors argued that the Opisthobranchia are therefore not a monophyletic group and can no longer be accepted as a valid taxon. The opisthobranchs are now included within the subclass Heterobranchia, although many manuals and websites still use the old classification.
  A phylogenetic study published in November 2004, gave new definitions of the seven main lineages of the Opisthobranchia.
  However, in 2005, a study of rRNA gene sequences could not resolve monophyly versus paraphyly of the Opisthobranchia 
  Subsequent taxonomic classification in 2005 overturned the Opisthobranchia as a valid clade, reclassifying it as an informal group within the Heterobranchia. Accordingly, articles no longer use the term Opisthobranchia, replacing it with Heterobranchia to emphasise a different concept. By 2011, the "Opisthobranchia" were declared artificial and obsolete and were replaced by new phylogenetic hypotheses.
caption: Aplysia californica, a sea hare.
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